1. Field of the Invention
The present invention relates to a particle beam irradiation apparatus and a particle beam irradiation method and particularly to a particle beam irradiation apparatus and a particle beam irradiation method for cancer treatment by irradiating an affected area with a heavy particle beam such as carbon, a proton beam, or the like.
2. Description of the Related Art
Currently, cancer is the first leading cause of death in Japan and more than 300 thousand people die from cancer every year. In light of such circumstances, much attention has been paid to a particle radiotherapy using a heavy particle beam such as carbon, a proton beam, or the like having excellent characteristics such as high therapeutic effect, less side effect, less physical strain, and the like. According to this treatment method, a cancer cell is irradiated with a particle beam emitted from an accelerator, whereby the cancer cell can be killed while normal cells are less affected.
As this treatment method, currently available particle beam irradiation method is a method called a broad beam method. This broad beam method expands a particle beam to a beam diameter equal to or greater than an affected area size by a method called a wobbler method or a double scattering method. Then, a brass collimator called a shaping collimator is used to restrict an irradiation field, whereby the beam shape is substantially matched to the affected area shape. Moreover, the range of the beam is expanded by a beam range expansion apparatus called a ridge filter in a beam traveling direction (in a beam axis direction); the beam stop position is matched to the affected area shape (outline) at a deeper position by a polyethylene beam range adjusting apparatus called a bolus.
However, the above mentioned broad beam method cannot exactly match the beam to the affected area shape three-dimensionally, and thus is limited to reduce the effect on normal cells around the affected area. In addition, there is another problem that the shaping collimator and the bolus are fabricated for each affected area (as well as each irradiation direction to the affected area), and thus these remain as nuclear wastes after treatment irradiation.
In light of this, as a further advanced irradiation method for the particle radiotherapy, there has been developed a 3D irradiation method for zeroing in on cancer cells with higher precision by three-dimensionally irradiating the affected area in the body (see Patent Document 1 (Japanese Patent Laid-Open No. 2009-66106)).
This method virtually cuts a treated area into small rectangular solids having 3D grid points and irradiates each grid point. Such a 3D irradiation method allows the beam to be matched to the affected area with a good precision also in a beam axis direction without using a shaping collimator or a bolus, and thus can suppress normal cells from being exposed in comparison with a conventional 2D irradiation method.
This 3D scanning irradiation method irradiates an affected area as follows. First, in order to set the depth position from a body surface which the beam energy reaches, the irradiation beam energy is selected. Then, a scanning electromagnet is used to two-dimensionally scan a surface (slice) perpendicular to the beam axis at the depth in the X and Y directions to irradiate a corresponding slice of the affected area with beams. Then, when all regions of the affected area on the slice are scanned, the beam energy is changed such that the beam depth position is set to the next slice and scans the regions of the affected area on the slice in the same manner. Such an irradiation is repeated on all the slices obtained by slicing the affected area in the depth direction, until irradiation on the entire affected area is completed.
When treatment is performed, it is important in terms of ensuring treatment safety to confirm how the irradiation is performed. As one of the methods, a spot position monitor for checking the beam position as needed is provided at downstream of the scanning electromagnet. Examples of the spot position monitors include a monitor using an ionization chamber system dividing a signal electrode into a multistrip or a monitor using a multiwire proportional counter system.
However, information obtained from these monitors is just position information discretely indicating the center of each beam spot, and thus a continuous dose distribution formed as an overlapped spot beam shape cannot be obtained from the spot position monitor.
In fact, it is desirable for a doctor or an operator to be able to visually confirm a dose profile (a 2D distribution of dose in X and Y directions or a 1D distribution of dose in X or Y directions extracted from the 2D distribution) during irradiation. For example, if a dose profile is displayed for each slice, the doctor or the operator can perform treatment while confirming that the irradiation is correctly performed and thus the doctor or the operator can perform treatment with a feeling of safety. However, such a method of monitoring the dose profile during irradiation is not currently available.
In view of such circumstances, the present invention has been made, and an object of the present invention is to provide a particle beam irradiation apparatus and a particle beam irradiation method which can provide a visual and quantitative confirmation as to how irradiation is actually performed by monitoring a 2D or 1D distribution of dose during particle beam irradiation.